1.9 Translate Unidirectional Signals Using Open-Drain Outputs

Hello, and welcome to The Logic Minute. In this video, we will look at how to use an open-drain logic device to interface parts operating at different voltages.
In many systems, a digital device operating at one logic voltage needs to communicate with the device operating at a different voltage. In situations where this communication is only going in one direction and has a low data rate, an open drain buffer or inverter is a simple and effective solution.
Some common examples include driving an LED, transmitting an over temperature signal, or separating a battery voltage from a regulated voltage.
This is possible because the output of an open-drain device only contains an end-channel MOSFET connected to ground-- highlighted here in red. And can only drive the output low.
An external pull-up resistor is required to produce a logic high at the output. This feature allows the output of this device to operate at a different voltage than the internal logic.
When selecting a part, it's important to check the datasheet to be sure that it will work for your application. In the recommended operating conditions table, the supply voltage controls the threshold for the input of the device and should match the input signal's voltage.
The maximum input voltage indicates if the supply value can be exceeded. In this case, the input voltage is not related to the supply voltage indicating an overvoltage tolerant input. Finally, the maximum output voltage indicates the highest voltage that the output pull-up resistor can be connected to.
Thank you for watching. Please explore the other videos and training material on our site. And if you have any questions, come over to the E2E forums to ask us directly.

Details

Date:
September 26, 2018

Many systems have multiple logic voltage nodes, and it is fairly common to need to communicate between them. This video discusses the method and limitations of translating using an open-drain logic device.